This application claims priority to Japanese patent application No. JPAP11-259126 filed on Sep. 13, 1999 in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method and apparatus for optical writing, and more particularly to a method and apparatus for optical writing that is capable of effectively performing an accurate scanning.
2. Description of Background
An optical writing device has been developed that includes a light-emitting element array unit employing a plurality of light-emitting elements (i.e., light-emitting diodes (LED)) arranged in a line on substrates. This optical writing device is configured to cause the light-emitting elements to emit light to expose a charged surface of a photoconductive member, which is a writing object, so as to form an electrostatic latent image on the photoconductive member. With such an optical writing device, it is possible to write an image of various sizes. However, to write an A0-scaled image, for example, the optical writing device needs to have a light-emitting element array unit having a width of approximately 1 meter. Manufacturing of such a long light-emitting element array unit which may have a resolution of 400 dpi (a dot-pitch of 63.5 xcexcm), for example, requires expensive tools, such as a wide LED-chip mounter, a wide wire bonding machine, and so on.
In addition, a unit cost of self-focusing rod-shaped lens arrays (i.e., a SELFOC lens array (SLA)), correspondingly provided relative to the light-emitting element rows, generally increases as the width increases.
Japanese Laid-Open Utility Publication No. 64-16342 (1989) describes an optical writing device which attempts to avoid the above-mentioned drawbacks. This optical writing device arranges several light-emitting element array units in a formation in order to make one longer light-emitting element array unit.
In this case, however, positions of the light-emitting element array units require accurate adjustments to write a straight line with dots. This can become problematic. That is, if a dot-pitch error is greater than approximately 5 xcexcm, an image will generally be reproduced with dirty marks of black lines or white lines. Yet, the dot pitch of the 400-dpi resolution is 63.5 xcexcm. Under this circumstance, connecting portions of the light-emitting element array units are sensitive to variations of environmental temperature, which is prone to increase due to actions of light emission by a large number of the light-emitting elements and so on. In other words, the merely-connected portions in the conventional device will be extended due to an increase of environmental temperature, causing a change of the dot pitch at each of the connecting portions.
More specifically, if parameters are set in such a way that a distance between connecting points of a connecting member for connecting a plurality of the light-emitting element array units is 20 mm, the connecting member is made of iron having a linear expansion coefficient of 0.000012 per degree, and an increase in temperature of 30 degrees, it will cause at least a dot-pitch error of 7.2 xcexcm, because an initial positioning error will be added so that an extent of the dot pitch error becomes greater. Therefore, a reproduced image will have the above-mentioned dirty marks around the connected portions of the light-emitting element array units.
Further, a casing for supporting the light-emitting element array substrates is usually made of a different material from that of the substrates and therefore, the linear expansion coefficients of the casing and the substrates are generally different. Moreover, the casing and the substrates generally have temperature distributions different from each other.
Therefore, a mere connection of the bodies of the light-emitting element array units will cause a large deviation of the dot pitch between the connected portions of the adjacent light-emitting element array units. In addition, a dot-pitch error in a sub-scanning direction orthogonal to the light-emitting element rows is prone to be caused also due to an increase in temperature.
The present invention provides a novel optical writing device. In one example, a novel optical writing device includes a plurality of light-emitting element array substrates, a plurality of connecting members, a plurality of self-focusing rod-shaped lens arrays, and a package case. Each of the plurality of light-emitting element array substrates includes a plurality of light-emitting elements arranged in a line for emitting light in a straight line. The plurality of light-emitting element array substrates are located parallel to each other and partly in an overlapping formation so as to be able to write on an area of a predetermined width in a direction in which the plurality of light-emitting elements, of each of the plurality of light-emitting element array substrates, are arranged in a line. Each of the plurality of connecting members connects two adjacent light-emitting element array substrates to each other around a position at which an operation for writing dots with the plurality of light-emitting elements is switched from one of the plurality of light-emitting element array substrates to a subsequent one of the plurality of light-emitting element array substrates. The plurality of self-focusing rod-shaped lens arrays are arranged in positions corresponding to positions of the plurality of light-emitting elements of the plurality of light-emitting element array substrates. The package case packages the plurality of light-emitting element array substrates and the plurality of self-focusing rod-shaped lens arrays into one device.
The above-mentioned plurality of light-emitting elements and the above-mentioned plurality of self-focusing rod-shaped lens arrays may be arranged so that the light emitted from the light-emitting elements of the plurality of light-emitting element array substrates come into focus in a straight line along a main scanning direction on a writing surface of a writing object rotating in a sub-scanning direction.
The above-mentioned plurality of light-emitting elements and the above-mentioned plurality of self-focusing rod-shaped lens arrays may be arranged so that each light axes of the light emitted from the light-emitting elements of the plurality of light-emitting element array substrates passes through the plurality of self-focusing rod-shaped lens arrays and is normal to a writing surface of a writing object.
The present invention further provides a novel method for optical writing. In one example, a novel method for optical writing includes the steps of providing, placing, connecting, arranging, and packaging. The providing step provides a plurality of light-emitting elements to a light-emitting element array substrate for emitting light in a straight line. The placing step places a plurality of the light-emitting element array substrates, each having the plurality of light-emitting elements, at positions so as to be parallel to each other and in a partly overlapping formation to be able to write on an area of a predetermined width in a direction in which the plurality of light-emitting elements of each of the plurality of the light-emitting element array substrates emit the light in a straight line. The connecting step connects two adjacent light-emitting element array substrates of the plurality of the light-emitting element array substrates to each other around a position at which an operation for writing dots with the plurality of light-emitting elements is switched from one of the plurality of the light-emitting element array substrates to a subsequent one of the plurality of light-emitting element array substrates. The arranging step arranges a plurality of self-focusing rod-shaped lens arrays at positions corresponding to positions of the plurality of light-emitting elements mounted on the plurality of the light-emitting element array substrates. The packaging step packages the plurality of the light-emitting element array substrates and the plurality of self-focusing rod-shaped lens arrays into one device.
The above-mentioned steps, for providing the plurality of light-emitting elements and for arranging the plurality of self-focusing rod-shaped lens arrays, may be performed so that the light emitted from the light-emitting elements of the plurality of the light-emitting element array substrates comes into focus in a straight line along a main scanning direction on a writing surface of a writing object rotating in a sub-scanning direction.
The above-mentioned steps for providing the plurality of light-emitting elements and for arranging the plurality of self-focusing rod-shaped lens arrays may be performed so that each light axes of the light emitted from the light-emitting elements of the plurality of the light-emitting element array substrates passed through the plurality of self-focusing rod-shaped lens arrays and is normal to a writing surface of a writing object.